The present invention relates to molding compositions which exhibit improved warp resistance, high deflection temperature under load (DTUL) and high impact strength in the molded article. More particularly, the invention relates to compositions comprising a thermoplastic resin selected from the group comprising a high molecular weight linear polyester, mixtures of high molecular weight linear polyesters, mixtures of a high molecular weight linear polyester and high molecular weight block polyester and mixtures of high molecular weight linear polyesters and high molecular weight block copolyester, an impact modifier and a filler material selected from the group comprising finely divided surface treated clay and amorphous silica and a process for improving the warp resistance of the polyester resin compositions without impairing other desirable properties such as strength, modulus and heat deflection temperature.
With the development of molecular weight control, the use of necleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection moldable compositions. Poly(1,4-butylene, terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Workpieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.
It has been previously disclosed in co-pending application Ser. No. 662,910, filed Mar. 1, 1976, assigned to the same assignee as herein, that glass reinforced thermoplastic compositions of a polycarbonate resin and poly(1,4-butylene terephthalate) can be molded into articles having greater resistance to warpage and/or improved DTUL, in comparison with glass fiber reinforced poly(1,4-butylene terephthalate) resins. It is further disclosed in co-pending application Ser. No. 753,861, filed Dec. 23, 1976, assigned to the same assignee as herein, that zinc stearate when added to polyester polyblends elevates notched Izod impact strength, while maintaining unnotched impact strength, flexural strength and tensile strength and dramatically reduces sample-to-sample variability in elongation. Also, it is disclosed in co-pending application Ser. No. 753,863, filed Dec. 23, 1976, and assigned to the same assignee as herein, that glass fibers in combination with a mineral filler provide molded articles with improved DTUL and/or reduced warpage. In addition, it is disclosed in co-pending application Ser. No. 747,635, filed Dec. 6, 1976, assigned to the same assignee as herein, that compositions comprising poly(butylene terephthalate), poly(ethylene terephthalate), a polycarbonate and glass fiber have increased DTUL and/or reduced warpage. It is disclosed in co-pending application Ser. No. 778,945, filed Mar. 18, 1977, that blends of a poly( 1,4-butylene terephthalate) resin and a polycarbonate resin reinforced with fibrous glass, when admixed with a small amount of zinc stearate possess even less inherent warpage in the molded article and good moldability when compared with compositions of glass fiber reinforced poly(1,4-butylene terephthalate).
It has also been previously disclosed in co-pending application Ser. No. 866,009, filed Dec. 30, 1977, and assigned to the same assignee as herein, that high molecular weight linear polyester, such as poly(1,4-butylene terephthalate) and the like, either reinforced or unreinforced, can be made flame-retardant and exhibit improved arc resistance when admixed with a flame-retarding amount of a flame-retardant, such as an aromatic polycarbonate and a particulate material selected from the group consisting of finely divided mica and finely divided clay. It has also been previously disclosed in co-pending application Ser. No. 925,992, filed July 19, 1978, and assigned to the same assignee as herein, to provide improved reinforced thermoplastic molding compositions having improved resistance to warpage, high deflection temperature under load (DTUL) and high impact strength comprising a thermoplastic resin selected from the group consisting of a high molecular weight linear polyester, a mixture of high molecular weight linear polyesters, a mixture of high molecular weight linear polyester and high molecular weight block copolyester and a mixture of high molecular weight linear polyesters and high molecular weight copolyester, fibrous glass reinforcement, phlogopite mica and an impact modifier. In addition, there is also disclosed in U.S. Pat. No. 4,140,670 molding compositions and molded products of polybutylene terephthalate, reinforcing glass fibers, phlogopite mica and poly-butylene terephthalate-co-tetramethylene oxide) which have improved warp-resistance properties.
It has now been discovered that compositions of high molecular weight linear polyesters, optionally in admixture with block copolyesters, an impact modifier and a filler material selected from the group comprising finely divided surface treated clay and amorphous silica exhibit very little inherent warpage in the molded article and, in addition, are high in strength and exhibit a high modulus and heat deflection temperature.